Specifying Registers to be Placed in Input/Output Blocks (IOBs)
You can use the Registers to be Placed in IOBs (IOB) dialog box
which flip-flops and latches will be moved into IOBs when the device
is mapped. Only the first flip-flop or latch encountered in the path
from a pad can be moved into the IOB. Placing a flip-flop or latch
into an IOB moves it closer to the pad, so the pad to setup time decreases
but the delay from the IOB flip-flop or latch to the next synchronous
To Specify Registers to be Placed in IOBs
- Open the Registers to be Placed in IOBs (IOB) dialog box.
- In the Timing Constraints panel, select Miscellaneous > Registers to be Placed in IOBs and in the Workspace, right click the
constraint to edit and select Edit Constraint to edit a constraint, or right click the bottom row of the table
and select Create Constraint to create a new
- Cross probe
from another program.
- To limit the number of selections in the Available
registers for IOB packing list, enter a filter string
in the Filter box and click Find. For example, to list only registers containing the string "b/Q,"
type *b/Q* in the Filter box. The filter is not case sensitive.
- In the Available registers for IOB packing list, select the registers to add to IOBs.
- Click the Add button.
Selected registers will be moved to the Registers
for IOB packing list.
- Click Create to
create the specified constraint without closing the dialog box, or OK to create the constraint and close the dialog box.
Specifying registers to be placed in IOBs creates an IOB
constraint for each selected flip-flop or latch. The new constraint appears
in the current Workspace table.
For more information about this constraint, see the Constraints Guide in the
online document collection
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